A marking ink comprising titanium dioxide or a mixture of titanium dioxide and other colorants dispersed in a vehicle has been used in marking pens. These marking pens are generally designed so that the marking ink is supplied from an ink reservoir in the casing thereof through a valve mechanism to a writing member such as a bundle of fibers or a hollow plastic bar. Hence, it is necessary for the marking ink to have a very low viscosity of about 1/10,000 to 1/50 that of a conventional ink, i.e., 50 centipoises (cps) or less at room temperature, so that it can smoothly flow out of the writing member. Titanium dioxide, however, which has a very high specific gravity compared with organic components present, is very difficult to disperse stably in a vehicle of such low viscosity. That is, the precipitation and separation of titanium dioxide in the marking ink inevitably occurs in a relatively short period of time after the preparation thereof.
In the marking pen as described above, a hard ball with a diameter of about 5 to 10 mm is placed in the ink reservoir. When used, the marking pen is shaked and the marking ink is stirred by the action of the hard ball to re-disperse the titanium dioxide. This overcomes the above problem of precipitation and separation of titanium dioxide to a certain extent. If, however, the marking pen is allowed to stand for a long period of time without being used, caking of precipitated titanium dioxide occurs and the hard ball sinks in the titanium dioxide cake. Thus, for re-dispersion of the titanium dioxide it is necessary to shake the marking pen very vigorously. If the marking pen has been used, the action of the hard ball does not extend to the marking ink which has entered the writing member through the valve mechanism from the ink reservoir and has remained therein. Thus, if the marking pen is allowed to stand after being used, titanium dioxide precipitates in and separates from the marking ink, and blocks the capillary channel through which the marking ink flows, causing the phenomenon of inking failure on re-use.